Melanin is one of a very few examples of natural organic semiconductors and was demonstrated to be such in the early 1970s. Melanin is thus a desirable natural, environmentally friendly material with many known applications for the electronics industry. Melanin can be used to produce a wide variety of biologically friendly electronic devices and batteries used in applications such as medical sensors and tissue stimulation treatments.
Many metazoans and microorganisms form melanin naturally. Because the concentration of melanin in these organisms is generally low and melanin is very insoluble, melanin extraction is inefficient and natural melanin is expensive. It is known in the art that the yield of melanin in a microbial culture can be increased by using chemically defined culture media, targeted extraction from natural populations, culturing pure strains, mutation and selection, genetic modification, and by spiking culture media with melanin precursors such as tyrosine and phenylalanine.
Melanins (such as eumelanin, pheomelanin and pyomelanin) are natural polyphenols produced by living cells. Pyomelanin is a negatively charged hydrophobic polymer of imprecise structure and size (Turick et al., 2003, 2009). It is present in fungi (Nosanchuk and Casadevall, 1997; Carreira et al., 2001; Schmaler-Ripcke et al., 2009), but also in many bacteria such as species of Pseudomonas (Yabuuchi and Ohyama, 1972; Arai, 1980), Legionella (Chatfield and Cianciotto, 2007) and Shewanella (Turick et al., 2008). Unlike the well-known eumalanin, which is produced from dihydroxyphenylalanine (DOPA), pyomelanin is metabolically derived from homogentisic acid (HGA), which upon elimination from cells autooxidizes and polymerizes as pyomelanin (David et al. 1996; Chatfield and Cianciotto 2007; Schmaler-Ripcke, 2009; Yabuuchi and Ohyama, 1972; Ruzafa et al., 1994; Kotob et al., 1995). In cells, pyomelanin is often associated with proteins (albeit melanin associated proteins are relatively little studied), and it is more concentrated in the outer cell envelopes such as the lipopolysaccharide layer and cell capsule (Turick et al., 2003).
The primary role of pyomelanin in living cells remains debated as melanins were proposed to play various roles in different species. Melanins can alter the electrical charge of a cell, especially when the polysaccharide capsule is small or absent (Nosanchuk and Casadevall, 1997). In Cryptococcus spp. the expression of pyomelanin is correlated with virulence (Kwon-Chung, 1982). In Legionella, pyomelanin increases resistance to light (Steinert et al., 1995). An antioxidant role for pyomelanin has been often proposed and has been demonstrated in Burkholderia sp. (Boles et al., 2004; Boles and Singh, 2008) and Methylococcus thermophilus (Sokolov et al., 1992). Pyomelanin confers Legionella ferric reductase capabilities (Chatfield and Cianciotto, 2007) and may help cells reduce, immobilize or chelate metals (Chatfield and Cianciotto, 2007; Turick et al., 2008; Nyhus et al., 1997). Pyomelanin may also bind and help recycle soluble electron shuttles such as riboflavin or may be used to transfer electrons toward solid phases (Marsili et al., 2008; Turick et al., 2009). The capacity of melanins to transfer electrons is derived from their ability to change the state of their monomers between quinone, semiquinone and hydroquinone (FIG. 1) (McGinness et al., 1974; Turrick et al., 2010). Because melanins have broad energy absorbing properties, their capacity to exchange electrons are influenced by many types of energy sources, including ionizing radiation, UV light, visible light, IR light and heat (Dadachova et al., 2007). It was proposed that in nature pyomelanin may also serve as a terminal electron acceptor (Turick et al., 2008), electron shuttle (Arai et al., 1980; Keith et al., 2007), or conduit for electrons (Turick et al., 2010).
Due to the complex architecture and broad size range of melanin the chemical entourage of the various quinone centers vary within a melanin polymer. Hence, their redox properties also vary, albeit they can exchange electrons within and between melanin polymers. For this reason, rather than having a narrow redox potential (Eo) as most simple redox chemicals do, melanin shows a broad redox potential profile. The discharge or recharge of electrons from some quinone centers is likely followed by re-partition of electrons and protons within the polymer until equilibrium is reached. Because most redox transformations involving melanin in nature occur at low redox potential (dEo<2 V), the variation in the energy level associated with electron exchanges is small relative to the strength of the covalent bonds which hold the quinone structure together. Henceforth, it can be predicted that as long as charging potentials remain small (say ≦2 V) melanins can be charged:discharged with electrons numerous times without significant alteration of the basic structure. A set of usage conditions can be found (with regard to redox potential and depth of charging/discharging) where the extreme electron-load phases of melanin (i.e. quinone and hydroquionone) are sufficiently stable to allow a melanin based battery to be fully charged or discharged without significant effects on the melanin' stability. Based on these properties, melanins can be used to replace heavy metals as electron storage substrates in the construction of long life, deep cycle rechargeable batteries. Energy storage in melanin is ecofriendly because melanin is composed only of elements (i.e. carbon, oxygen and hydrogen) that are abundant and easily recycled in near earth surface ecosystems. Henceforth, an economy using melanin-based energy storage will not become resource limited, nor will it compete with materials needed by other economic activities.
Relative to other types of melanin, pyomelanin has the benefit of being produced in microbial cultures, has narrow range molecular size, 12,000-14,000 MW (Turrick et al., 2002), and it is easier to dissolve and purify. These properties make pyomelanin an excellent choice for applied technologies that require predictable product quality and predictable redox chemistry. In contrast, eumelanin and pheomelanin are only available in low supply and eumelanin is highly variable in size (it may have a molecular mass as large as 106 g/mol, making some eumelanin fractions highly insoluble). Last but not least, the cost of producing melanin is important in evaluating its sustainability for energy storage devices. Commercial eumelanin is extracted from hair and squid or polymerized from DOPA-related monomers and presently costs between $300 and $600 per gram, which makes it unsuitable for most economical applications. In contrast, pyomelanin extracted from microbial cultures can be produced in large amounts and at considerably lower costs.
Pyomelanin and pyomelanin-related molecules can be produced in various ways. Most frequently proposed methods are: direct extraction from microbial biomass, enrichment in culture media spiked with tyrosine or phenylalanine, induction of genetically modified microorganisms and chemical oxidation of homogentisate. Microorganisms may produce between 1 and 115 fg of pyomelanin per cell (though a 1-10 fg yield is more common) depending on strain and physiological state (Turick et al., 2003; Turick et al., 2008; Turick et al., 2002). Most cell cultures will produce approximately 0.03-0.3% pyomelanin relative to dry weight (DW). Obtaining higher melanin yields requires using pure cultures, inducing agents, long incubation times, expensive media and controlled growth conditions. The biomass obtained in bacterial cultures may vary between 0.23 g DW/L in batch media without manipulated conditions, and 37.8 g DW/L in rich media under controlled conditions (Soini et al., 2008). Hence, simple batch cultures may produce 0.069-0.69 mg pyomelanin L−1, while complex controlled cultures may produce as much as 1.13 g pyomelanin L−1 (albeit obtaining such extremely high yields is costly).
Citation or identification of any reference in Section 2, or in any other section of this application, shall not be considered an admission that such reference is available as prior art to the present invention.